The prosecution in the murder trial of George Zimmerman wrapped up its case after a dramatic day of testimony Friday.

Both Travyon Martin's mother and brother told the jury that the screams for help that can be heard in the background on a 911 call came from the 17-year-old.

Before the defence began its case, defense lawyer Mark O'Mara argued Zimmerman should be acquitted, saying prosecutors hadn't proved their case.

"What is before the court is an enormous amount of information my client acted in self-defense," Mr O'Mara said told Judge Debra Nelson.

Before the state rested, Martin's mother, Sybrina Fulton, sat expressionless on the witness stand while prosecutors played the 911 recording, in which high-pitched wails can be heard as Zimmerman's neighbour urges a dispatcher to send police quickly. Moments later on the call, there is a gunshot and the crying stops.

She was followed on the stand by her son Jahvaris Fulton, Martin's 23-year-old half brother, who also testified the cries came from Martin.

Identifying the voice could be critical to the case because it could help the jury determine who was the aggressor during the scuffle that ended with Zimmerman killing Martin. Zimmerman's father has claimed it's his son yelling.

Zimmerman, the former neighbourhood watch volunteer charged with second-degree murder, has said he shot the teen in self-defence during a fight.

Late in the afternoon, the prosecution rested after calling 38 witnesses over two weeks.

When introducing herself to jurors, Sybrina Fulton described having two sons, one of whom "is in heaven."

During cross-examination, Zimmerman lawyer Mark O'Mara suggested - haltingly, in apparent recognition of the sensitivity of the questioning - that Sybrina Fulton may have been influenced by others who listened to the 911 call, including relatives and her former husband.

After the mother and brother testified, the doctor who performed an autopsy on Martin took the stand. Associate Medical Examiner Shiping Bao started describing Martin as being in pain and suffering after he was shot, but defence lawyers objected and the judge directed Mr Bao away from that line of questioning.

He later estimated that Martin lived one to 10 minutes after he was shot, and said the bullet went from the front to the back of the teen's chest, piercing his heart.

"There was no chance he could survive," Mr Bao said.

With jurors out of the courtroom, Mr Bao acknowledged under defence questioning he had changed his opinion in recent weeks on two matters related to the teen's death - how long Martin was alive after being shot and the effect of marijuana detected in Martin's body at the time of his death.

Mr Bao said last November that he believed Martin was alive one to three minutes. He also said Friday that marijuana could have affected Martin physically or mentally; he said the opposite last year.

The judge ruled before the trial that Martin's past marijuana use couldn't be introduced, and so the jury did not hear Mr Bao's opinion about the drug's effect.

Zimmerman lawyer Don West questioned why Martin's hands weren't covered to preserve evidence on his fingers and why it took three hours to remove the body from the scene. Mr West and Mr Bao talked over each other at several points, prompting the judge to tell everyone to speak one at a time.

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